The invention relates to reels or spools, and more particularly to improved plastic reels or spools for storing fine magnet wire or monofilament, although other uses are contemplated. Such reels or spools typically comprise a core provided with a pair of axially spaced annular flanges. It is common in the industry to prepare such reels by molding identical half sections which can be joined to provide a complete reel. The most convenient method of joining such half sections is by securing together mating core sections. In commercial practice these mating core sections are usually joined by use of a solvent adhesive. A typical such prior art spool is shown in U.S. Pat. No. 3,966,139 (Terpak, June 29, 1976) and U.S. Pat. No. 3,942,741 (Hussar et al, Mar. 9, 1976). While some efforts have been made to avoid use of adhesives and to employ welding, particularly ultrasonic welding, in assembling such reels, this has not achieved any commercial success due to the difficulty in transmitting the ultrasonic welding energy to the parting line of the mating core sections. For example, U.S. Pat. No. 4,607,806, (Yealy, Aug. 26, 1986) shows a welder head welding the end of a core to a reel flange. However, Yealy does not involve two identical plastic core sections which are joined adjacent the center of the core connecting the two ends of the reel. U.S. Pat. No. 3,334,841, (Burhop, Aug. 8, 1967) also mentions welding but does not describe how the welding is achieved.